SAN FRANCISCO — Rivalries are often overrated. A few, though, are very underrated.

Example of an overrated rivalry: Twinkies vs. Ho Ho’s. It made no sense. Both were made by the same company — which is now dissolving into a pile of gooey sugar debt. That will never happen with Apple vs. Microsoft, or Ohio State vs. Michigan.

Example of a very underrated rivalry: The 49ers vs. the Chicago Bears. The two classic National League Football franchises are not in the same division, so they don’t face each other annually. But whenever they do meet, the two teams invariably produce gripping moments.

Expect more of the same Monday night. It’s a quasi-historic event. Given the NFL scheduling formula and the pace of Santa Clara Stadium construction, this will likely be the final time that the two teams meet at Candlestick Park. The Bears won’t mind. They haven’t won at Candlestick since 1985 and are 0-7 in San Francisco since.

It says here, then, that a good old-fashioned smashmouth game could be at hand. The Bears (7-2) and 49ers (6-2-1) are leading their divisions. Jim Harbaugh, the 49er coach, has proclaimed that “the football world will be watching this one.”

Also watching will be Jay Cutler, the Bears’ starting quarterback. He is out after sustaining a concussion last week, replaced by backup Jason Campbell. Alex Smith, the 49ers’ starter, is dealing with his own concussion. But he’s supposed to go.

Might not matter. Quarterbacks are frequently bit players in this rivalry. Same this time. The 49ers and Bears of 2012 both seem to rise and fall on their defensive performances. When you think about this rivalry, the thoughts usually involve hard hitting — and/or gum throwing. In 1987, former Bears coach Mike Ditka was so enraged by a 41-0 loss to the 49ers at the ‘Stick that he hurled his chewing gum into the seats toward a group of fans that had been taunting him, striking one in the head.

I remember that cold December night and the hubbub that followed. San Francisco police confiscated the green wad that Ditka had allegedly tossed, saying they were contemplating charges against him. But in the end — despite evidence of “the telltale smoking gum,” as labeled by cheeky Chicago columnist Bob Verdi — the legal system decided other things were more important.

The 49ers-Bears games, though, always traffic in such emotion. On the rivalry scale, I would even place it above 49ers-Rams. As proof, I present my hand-selected 10 most memorable games in the series:

Sept. 24, 1950, Kezar Stadium. This was the 49ers’ second game as an NFL team after four seasons in the old All-America Football Conference. As such, it was a huge deal. The AAFC was demeaned as a “minor league” by the NFL stalwarts. The Bears thought they’d proved the point with a 32-20 victory. But the 49ers won four of the next seven games in the series. From 1950 through 1966, the 49ers and Bears were in the same NFL division and played twice a season.

Dec. 12, 1965, Wrigley Field. Gale Sayers, the future Hall of Fame running back, had his greatest game as a pro by scoring six touchdowns against the 49ers in a 61-20 victory. If you have seen any NFL Films production about Sayers, you have seen highlights of this game.

Nov. 13, 1966, Wrigley Field. This was the only tie in the series, with a 30-30 final score. The 49ers’ Ken Willard rushed for three touchdowns and John Brodie threw for 327 yards while leading the team to a fourth-quarter comeback after they trailed 30-20.

Nov. 10, 1968, Wrigley Field. This might have been Sayers’ lowest moment as a pro. He mangled his knee while being tackled during a 27-19 Bears victory. He was never the same. With today’s medical technology, Sayers probably could have recovered to play well for many more seasons. Instead, he played his last NFL game in 1971 and retired at age 29.

Oct. 28, 1979, Candlestick Park. Bill Walsh’s first 49er team, which won only two games, nearly pulled off a third victory before falling to the Bears, 28-27, on a late touchdown pass from Mike Phipps to James Scott. But the near miss was a harbinger of Walsh’s future success.

Sept. 13, 1981, Candlestick Park. The 49ers had lost their season opener the previous week at Detroit, bringing back the mantra of “same old Niners.” But in a 28-17 victory over the Bears, Joe Montana threw three touchdown passes and set the team on the path toward a 13-3 season and the 49ers’ first Super Bowl title.

Dec. 14, 1987, Candlestick Park. The gum game. See above. Chew on it a while. Then please dispose of the wrapper.

Jan. 8, 1989, Soldier Field. On a frostbite day, the underdog 49ers laid a stunning and decisive 28-3 loss on the Bears in the NFC Championship game. I vividly recall Walsh’s flushed and frozen face in the locker room afterward and the peeved countenance of Ditka.

Oct. 28, 2001, Soldier Field. This might have been the start of the end for Steve Mariucci as head coach. The 49ers had a 28-9 lead in the third quarter, but the Bears roared back to tie the game and win in overtime, 37-31, on a 33-yard interception return by Mike Brown. Afterward, 49er receiver Terrell Owens implied that Mariucci had eased up with a big lead because of his friendship with Chicago coach Dick Jauron. The charge was preposterous, but the Mariucci-Owens relationship was never the same.

Nov. 12, 2009, Candlestick Park. In a Thursday night game, Mike Singletary faced his former team for the only time as a head coach and won, 10-6, thanks to a Frank Gore touchdown, a Joe Nedney field goal and solid defense.

It would take a pretty unforgettable game for Monday’s contest to break into that list. But it won’t surprise me if it happens. With 49ers-Bears, the unforgettable often turns out to be very gettable.

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